Silicon is a very important material in the electronics industry and the solar cell industry. Currently, high-purity silicon for use in the solar cell industry can be produced using metallurgical methods. However, silicon of metallurgical quality results in solar cells with limited efficiency. Methods for chemical vapor deposition (CVD) must be used to produce silicon of sufficient purity that gives higher efficiency for solar cells in addition to applications within the electronics industry. Silicon waste from the electronics industry has been used for producing solar cells. However, the growth of the solar cell industry has caused shortage of silicon. The world's huge need for environmentally friendly energy, in addition to the continually growing importance and spread of the electronics industry, involves a great need for equipment which contributes to the production of high-purity silicon at low costs.
The Siemens process is the most widespread method for CVD used to produce polycrystalline silicon. More precisely, a halosilane, like trichlorosilane, deposits onto a resistance heated rod. The energy requirement is high. A more detailed description of the process is described in the patent publication U.S. Pat. No. 3,979,490.
Another method for CVD is fluidized bed pyrolysis, wherein silicon seed particles are surrounded by and kept in an ascending gas flow, as the gas flow comprises silicon-bearing gas, from which silicon can deposit onto the seed particles. The advantage using this fluidized bed is the vast surface area onto which silicon can deposit, which enables the possibility of increased and continuous production as well as lower energy consumption. However, a practical and simple way to bring out particles that has grown sufficiently large is hard to achieve in practice. More precisely, it is hard to control the particle size in a fluidized bed reactor, and it is very hard to control the distribution of the particles in the operating reactor. The uneven distribution of the particles affects the flow state, which again affects the temperature distribution and the depositing of silicon. The method requires adding new particles from outside the reactor or small particles that form in the reactor during operation. The drainage of large particles in the bottom of the reactor as well as the addition of small particles that should increase in size requires at the same time controlling many parameters, which has proven to be very hard in practical operation over time. A common problem in CVD reactors is that particles grow together and gradually block the fluidization. Furthermore, the decomposition of silane or halosilane in a heated fluidized bed reactor can cause deposits of fine particles, which is undesirable, as accessories for particle separation and subsequent melting are required. This causes comprehensive handling and increased risk of contamination and loss of material.
The biggest problem in CVD reactors, however, is usually unwanted deposition of silicon onto the inner surfaces of the reactor and nozzles, which causes the nozzles and the reactor volume to clog. To prevent such undesirable deposits of silicon, the seed particles in a fluidizing reaction zone must be kept at a reaction temperature while the temperature of the reactor walls and nozzles must be kept significantly lower, which is extremely hard to achieve, especially when heating from outside the reactor. The problem is discussed in the patent publication U.S. Pat. No. 4,818,495, column 2, line 40, to column 3, line 20, and in the patent publication U.S. Pat. No. 5,810,934. A technique that is deployed to be able to loosen silicon from the reactor walls and its nozzles, is to apply a non-stick coating to the surfaces, which simplifies the subsequent manual or automated removal of silicon deposits.
A thorough description of methods for CVD with a fluidizing bed and appurtenant equipment as well as operation parameters for the production of silicon, including gas mixtures, temperatures for deposits and problems and limitations concerning this, can be found in the patent publications U.S. Pat. No. 4,818,495 and U.S. Pat. No. 5,810,934 which contain detailed information.
There is a need for an alternative technology which is advantageous with regard to one or more of the problems mentioned above.